Professionally administering Linux is a very complex matter: it requires advanced skills on the kernel and on a lot of tools and utilities that are often combined in pipelines and scripts.

Professionals having a thorough understanding of the kernel know how to get optimal performance for specific workloads by tuning the right “knobs” and to what degree to do it. In addition to that they know what commands and tools should be used to gather all the information necessary to fix performance issues. This of course requires having a thorough understanding of the kernel scheduler, of memory allocators, of the virtual memory, the Virtual File System and so.

As anybody would expect, skilled professionals thoroughly know the Linux distribution they use – being able to install and remove software packages is absolutely not enough. They should know:

the file system standard layout used by the distro – so to honour it when installing custom software
how to create custom packages (such as RPM) to ease the installation of custom application and scripts
the peculiarities of the initialization process specific of the distribution, in order to sort out boot issues that may arise sooner or later
how to perform automated installations
how to use filesystems, partitioning and Logical Volume Management utilities to apply changes while keeping online the system

And of course a lot of other stuff.

X Window Tutorial – X Display Server HowTo And Cheatsheet

Posted on April 17, 2024April 18, 2024 by Marco Antonio Carcano

Even just because of its very long service lifetime, it is certainly worth the effort to have a look at this amazing piece of software. It does not matter if we are about to switch to Wayland (that by the way cannot completely replace the whole X Window System - think for example to XDMCP): the truth is that systems running X Windows will stay here for 10 years more, so 'm sure it is still worth the effort to have a good understanding of it. The "X Window Tutorial - X Display Server HowTo And Cheatsheet" post provides you with all the necessary skills to become an expert on this amazing piece of software that really made the story of UNIX and Linux.

X window – display manager (remote XDMCP) and thin clients

Posted on April 2, 2024April 18, 2024 by Marco Antonio Carcano

X Window System is a legacy (1984) windowing system, that is a system providing the basic framework for a GUI environment initially developed as part of Project Athena at MIT. The currently available X protocol version is 11 (dated 1987!) - hence often people refer to X Window as "X11''.

It is really time I wanted to write a few posts on this amazing software that really made the story of IT on the UNIX and Linux platforms: it is one of my favorite software ever, it was really impressive in those years being able to run an application on a host while being able to just render on a remote display.

In the X window - remote display manager (XDMCP) and thin clients we see how to reproduce on a modern Linux distribution a very common setup of the late '80 and early '90: a remote server running applications rendered on X Window based thin clients.

Apache HTTPd With Mutual TLS and OCSP Stapling

Posted on March 6, 2024March 7, 2024 by Marco Antonio Carcano

TLS peers can verify if a certificate was revoked by checking the CRL (very old and very poorly performing method with lots of shortcomings) or query the OCSP endpoint of the CA that issued the certificate.

However this design still has a shortcoming: what happens if for any reason the OCSP endpoint is unreachable (by accident or by anything caused by the evil people out there)? The outcome is a security risk - if the policy is to deny connection if the OCSP status cannot be checked, you risk to disserve.

Conversely, if the policy is that OCSP status check is a nice to have, there’s a risk that, if a revoked certificate has been stolen by the evils out there, they can just prevent your client to query the OCSP server and hijack the connection to a rogue TLS server managed by them that uses the stolen revoked certificate. To mitigate this you can set up OCSP stapling, which consists of prefetching OCSP responses and attaching them to the X.509 certificate.

In the "Apache HTTPd With Mutual TLS and OCSP Stapling" post we see not only how to configure an Apache server to provide a stapled certificate, but also how to set up mutual TLS authentication, seeing in action what happens when a certificate is revoked.

Cloudflare’s certmgr tutorial – a certmgr howto

Posted on February 26, 2024 by Marco Antonio Carcano

While running a Public Key Infrastructure (PKI), the maintenance workload due to enrolling new certificates and renewing the existing ones can quickly become overwhelming. Dealing it manually is not only cumbersome: it is frustrating too. Luckily there are ways to automate the enrollment process by providing online Registration Authority endpoints.

Cloudflare's PKI and TLS Toolkit provides both an online Registration Authority as well the client software that can be used to automatically enroll new or renew existing certificates. The aim of the "Cloudflare's certmgr tutorial - a certmgr howto" blog post is ti show how quick and easy is setting up certmgr, the certificate monitoring and automatic enrolling facility provided by Cloudflare.

The operating environment used in this post is Red Hat Enterprise Linux 9 or Rocky Linux 9 - using a different environment may lead to having to adapt or even change things.

Indirect CRL generation, CRL validation and OCSP validation

Posted on June 21, 2023February 24, 2024 by Marco Antonio Carcano

In the "OpenSSL CA tutorial - a full-featured OpenSSL PKI" post we set-up a full featured Public Key Infrastructure with Root and Intermediate Certificate Authorities, Indirect CRL and OCSP Responders. To have a go with that PKI, we also generated an Extended Validation (EV) certificate ("/tmp/foo.crt").

In this post we are using the same PKI we set up in that post, the EV certificate we generated and we also generate a new Organization Validated (OV) certificate: the goal this time is showing how to deal with Indirect CRL generation, CRL validation and OCSP validation.

OpenSSL CA tutorial – A full-featured openssl PKI

Posted on June 1, 2023February 24, 2024 by Marco Antonio Carcano

OpenSSL is a full featured tool capable not only to generate keys and certificates, but also to provide every facility a PKI must have, such as indirect CRL and OCSP responders: these features, along with certificate's best practices such as the Certification Practice Statement (CPS), publishing CRL Distribution Points URL, OCSP Responders URL, CA Issuers URL, are the topics of the OpenSSL CA tutorial - A full-featured openssl PKI.